In rotating machines, in general, different forces are acting between rotor and stator. Some of the forces are due to loads or force applications on the rotating shaft. Such forces are typically mechanical forces. Other forces, e.g. magnetic forces, may appear as a result of non-perfect rotor and/or stator configurations. Such forces are typically carried by different kinds of bearings. Rotating machines typically have both radial and axial bearings, of which either or both may be of a contact free type.
Some rotating electrical machines according to prior art utilizes a principle based on that a control flux is added onto the normal bias stator flux in a stator. There is an additional stator winding that enables the provision of different fluxes to different parts of the stator. The difference in stator current at different positions, in a circumferential direction, means that the airgap flux density varies along the airgap. The varying airgap flux density gives rise to a varying force between the rotor and the stator.
One example of a machine utilizing stator control windings to attenuate flexural rotor vibrations is disclosed in “Attenuation of Harmonic Rotor Vibration in a Cage Rotor Induction Machine by a Self-Bearing Force Actuator” by A. Laiho et al, in IEEE Transactions on magnetics, vol. 45, No. 12, December 2009, pp. 5388-5398. In the published International Patent Application WO 03/032470 A1, an electrical machine having capability to generate lateral forces is presented. The stator currents are distributed between different parts of the stator windings in different magnitudes, which gives rise to a resulting force between the rotor and the stator.
In the U.S. Pat. No. 5,053,662, electromagnetic damping of a shaft is disclosed. Input signals from a position sensor indicates that damping of a shaft of an electrical machine, with a permanent magnet or switched reluctance rotor, is desired. Stator windings, electromagnetically coupled with the rotor, are caused to be selectively energized to dampen the vibrations of the shaft, in response to the sensor indication.
A problem with such prior-art solutions is that the machine design has to be modified to comprise the additional stator control windings, which influences the performance of the entire machine. Furthermore, in large machines, the stator power that is necessary to control for achieving suitable forces is high, which requires complex and expensive power electronics.